1. Field of the Invention
The present invention relates to the field of power supply circuits.
2. Prior Art
In certain electronic equipment, it is desired or necessary to have two or more power supplies coupled to the equipment so that upon failure of one power supply to provide power to the equipment, another supply will automatically take over. The multiple power supplies may be of the same type or may be of different types. By way of example, redundant electronic supplies may be provided so that a failure of any one supply will not effect the operation of the equipment powered from the redundant supplies. In other cases, one supply might be an electronic supply powered from a municipal power supply, with a second supply being a battery backup supply. Thus, failure of the electronic supply or loss of municipal power will result in a takeover by the battery backup system.
In a redundant supply system, the multiple supplies may be of the same or of different voltages. If the voltages are different, typically the power is supplied to the load by the higher voltage active supply.
In such redundant power systems, it is usually desired to connect the redundant power supplies in parallel in a manner whereby the lower voltage power supplies cannot draw current from the higher voltage power supplies. For this purpose, some circuit must be connected between each of the multiple power supplies and the common load connection to potentially supply current to the load from any of the power supplies without any power supply drawing current from any other power supply. For this purpose, a simple diode connection between each power supply and the load has been used in the prior art. By way of example, FIG. 1a shows diodes D1 through Dn for coupling n power supplies, supplying voltages VIN1 through VINn to the input of the load Vin. Such diodes, passing current in one direction but not in the other, provide the desired function of supplying current to the input terminal VIN of the load while blocking all current flow between power supplies. Such circuits are referred to as input ORing circuits, as the load is powered by the first power supply or the second power supply or the third power supply, etc., whichever has the higher output voltage.
While FIG. 1a presents a prior art circuit for use on the positive side of the power supplies, FIG. 2a presents a corresponding circuit for use on negative power supplies or the negative side of power supplies. In addition, in some situations, it is desired to use a single power supply to power multiple loads, or at least potentially power multiple loads simultaneously without any one load being able to provide current back to any other load. FIG. 1b shows a diode circuit for such purpose for use on the positive power supply side, whereas FIG. 2b shows a corresponding circuit for use on the negative side of the power supply. Such circuits are referred to as output ORing circuits and might be used, by way of example, on battery chargers wherein a single charger is coupled to multiple rechargeable batteries. This allows multiple batteries to be charged by a single charger without any battery being discharged when the charger is off and a load on another battery discharges that other battery.
The use of diodes in this manner is simple, inexpensive and reliable. However, silicon diodes have a forward conduction voltage drop on the order of 0.7 volts. In the case of 12 volt lead acid batteries, such a diode voltage drop would result in approximately 5% of the power passing through the diode being dissipated in the respective diode. This may be tolerable in many applications. However, many present electronic systems operate at much lower voltages, such as 5 volts, 3.3 volts and even lower. At these voltages, a 0.7 volt drop represents a much higher percentage of power dissipation, thereby increasing the size of the power supplies needed, the cooling needed for the system, and making the equipment more expensive to operate. Also, while the use of Schottky diodes can somewhat reduce the power dissipation in the diodes, the reduction in the power dissipation is only partial.
Accordingly, in the prior art power MOSFETs have been used in place of the diodes. For instance, FIGS. 3a and 3b show n-channel MOSFET positive side power supply input ORing and output ORing circuits corresponding to FIGS. 1a and 1b, respectively, and FIGS. 4a and 4b show corresponding positive side input ORing and output ORing circuits using p-channel MOSFET devices. FIGS. 5a and 5b show n-channel MOS negative side input ORing and output ORing power supply circuits corresponding to those of FIGS. 2a and 2b, respectively, and FIGS. 6a and 6b show p-channel MOS negative side input ORing and output ORing power supply circuits corresponding to those of FIGS. 2a and 2b. In the prior art, the MOSFET devices are switched either by monitoring the input voltage, or both the input and output voltage.